ACTION OF THE FJbC^^ER LEAVES ON THE AIR. 139 



in the middle of the day, and hitter in the evening. — [Sprenge , Chemie, 

 II., p. 321.] During the night the oxygen has accumulated in these plants 

 and ibrned acids containing oxygen in excess (p. 127.) As the day ad- 

 vances this oxygen is given off; under the influence of light the acids are 

 decomposed, and the sourness disappears. 



In the juices of plants before the period of flowering, other acids are 

 met with besides the oxalic acid, though in much smaller quantity. As 

 the most important of these, however, occur jnore abundantly in fruits, 

 we shall consider the theory of their formation in the following section. 



§ 4. Of the chemical changes which take place bettveen the opening oftJie 

 flower and the ripening of the fruit or seed. 



The opening of the flower is the first and most striking step taken by 

 the plant towards the production of the seed by which its species is to be 

 perpetuated. That at this period a new series of chemical changes com- 

 mences in the plant is obvious from the following facts : — 



1°. That the flower leaves absorb oxygen and emit carbonic acid both 

 by day* and by night (p. 95.) 



2*^. That they also occasionally emit pure nitrogen gas. 



3°. That the juice of the maple ceases to be sweet when the flowers 

 are matured (Liebig,) and that, in the sugarcane and beetroot, the sugar 

 becomes less abundant when the plant has begun to blossom. 



These facts sufficiently indicate the commencement of new changes 

 in the interior of plants at this period of their growth. That such changes 

 go on until the ripening of the seed is also evident from these further ob- 

 servations : — 



1°. That the husk of the future seed, as in the corn-bearing grasses 

 (wheat, oats, &c.,) is filled at first with a milky liquid, which becomes 

 gradually sweeter and more dense, and finally consolidates into a mix- 

 ture of starch and gluten, such as is presented by the flour of different 

 species of corn. 



2°. That the fruit in which the seeds of many plants is enveloped is 

 at first tasteless, afterwards more or less sour, and finally sweet. In a 

 few fruits only, as in the lime, the lemon, and the tamarind, does a suf- 

 ficient quantity of acid remain to be sensible to the taste, when the seed 

 has become perfectly ripe. The acid and cellular fibre both diminish 

 while the sugar increases. 



3°. That fruits, while green, act upon the air like the green leaves and 

 twigs — but that, as they approach maturity, they also absorb or retain 

 oxygen gas (De Saussure.) The same absorption of oxygen takes place 

 when unripe fruits are j)lucked and left to ripen in the air (Berard.) 

 After a time the latter also emit carbonic acid. 



I. FORMATION OF THE SEED. 



In the case of wheat, barley, or other plants, which yield farinaceous 

 seeds, we have seen that previous to flowering the chief energy of the 

 living plant is expended in the production of the woody fibre of which 

 its stem and growing branches mainly consist ; and we have also been 

 able to understand, in some degree, how this woody fibre is produced 

 from the ordinary food of the plant. When the flower expands, how- 



• By day the absorption is the greater, but the bulk of the oxygen taken in is always 

 greater than that of the carbonic acid given off. 



